Switching power supplies are power converting circuits that are widely used in electronic devices such as televisions and computers. A switching power supply converts an AC (alternating current) input power to a DC (direct current) output power, and controls the output power by pulse-width modulation (PWM). A switching power supply adopts various protection function, e.g., over current protection (OCP), over voltage protection (OVP), and over temperature protection (OTP) to prevent the system from being damaged. The OCP limits the maximum level of the current of the switching devices by cycle-by-cycle current limiting.
FIG. 1 illustrates a conventional flyback-type switching power supply 100 which includes a bridge rectifier and an isolated DC/DC converter. The isolated DC/DC converter includes a transformer 111, an optical-coupler 113, a switch controller 115 and a switch 117, e.g., a metallic oxide semiconductor field effect transistor (MOSFET).
For the conventional switching power supply 100, if the input voltage changes from AC 85V to AC 265V, the maximum output power changes significantly due to the following two reasons. First, if the input voltage is relatively high, then the switching power supply 100 operates in a discontinuous conduction mode (DCM) and an inductor current error appears because of the switching-off delay which is caused by the driving stage of the switch controller 115. Second, if the input voltage is relatively low, the switching power supply 100 operates in a continuous conduction mode (CCM). The power transmission efficiency in CCM is lower than in DCM. As such, the maximum output power of the switching power supply 100 is significant greater if the switching power supply 100 receives a relatively high input voltage, which is undesirable. Currently, duty cycle compensation is adopted to maintain a stable maximum output power for a wide range of the input voltage. A drawback of this method is that the output power is affected by the inductance of the inductor in the circuit. Because the error of the inductance can be up to 20% of the rated value, it is difficult to adjust the inductance.
Furthermore, in a switching power supply, one or more safety capacitors (XCap) need to be installed between the two terminals of the AC input in order to suppress the electromagnetic interference (EMI). According to safety requirements, if the AC power source is unplugged, the voltage across the safety capacitor must be less than 37% of its peak voltage after one second. In other words, the voltage across the safety capacitor should rapidly decrease to a safe value to make sure the remaining electric charge will not harm a user.
FIG. 2 is a conventional resistor discharging circuit 200. As shown in FIG. 2, one or more safety resistors 203 are coupled between two terminals of the AC input to achieve quick discharging of the safety capacitor 201 if the AC power source is unplugged. A drawback of this solution is that the safety resistors will cause relatively large power dissipation.